1. Field of the Invention
This invention is related to transparent sheets formed from blends of polyethylene terephthalate and polycarbonate resins containing an antiblock agent, a process for their production, and containers formed therefrom.
2. Discussion of the Prior Art
Polyethylene terephthalate (hereinafter sometimes referred to as "PET") resins can be employed to prepare transparent film and sheet. Usually the resin is extruded into an amorphous flat sheet which is then biaxially stretched and thereafter heat set to impart a desired degree of crystallization to the sheet. Such biaxially oriented and crystallized products are strong and clear, but cannot readily be formed, such as by thermoforming, into containers since the process of biaxially stretching removes most of the extensibility of the sheet. If amorphous PET sheet is produced by rapid cooling of the molten sheet, a clear and transparent product may be obtained which is formable into containers. These containers may be used for lower temperature filling food packaging, at temperatures of from about 100.degree. to 140.degree. F. Blended with higher amounts of polycarbonate, the sheet can be formed and filled with foods at higher temperatures, for example, 150.degree.-180.degree. F.
Amorphous polyester sheet is also produced for meat and pharmaceutical packaging as well as warm and hot fill of syrups and other foods. These packages are often times formed with the well-known prior art process of thermoforming. This is where a blocking problem will often occur. In one case, preforms are stacked together then transported to another part of the plant where they are denested and filled with meat. Whereas no difficulties in separating the preforms ever occurred with the previously used polyvinyl chloride sheet, great difficulty was experienced in trying to separate the polyester preforms due to the sheet's tackiness. In another case, the lack of slip in the sheet caused cups of syrup to jerk on the filling machinery resulting in spillage. Because of these problems, it is necessary to produce a grade of polyester sheet with superior slip and block properties.
Probably the most commonly used additive for improving blocking properties is silica of one form or another, be it in polyolefins, PVC, or other plastic film. Usually, in the case of pellet feedstocks, such things as slip additives, colorants, and other additives are added in the form of pelletized concentrates of a masterbatch.
However, in the case of polyester, there are very few compounders with processing equipment which can handle the high temperatures required for PET, and, in particular the high intrinsic viscosity material (0.95). The use of extruders for compounding fine particulates, such as antiblock agents, into polyester does not work well if the polyester is in pellet form since not only do the two components settle out quickly in an extruder hopper, but the low melt viscosity of PET leads to a poorly dispersed concentrate. Some PET was ground to a fine powder which solved the settling out problem, but the dispersion in the concentrate after processing on a single screw two-inch extruder equipped with a mixing screw was still poor. Since PET is usually available only in pellet form, the additional pulverizing step creates an additional expense.
It is known in the prior art to add silica filler to one of the blended polymers, preferably the polycarbonate, as disclosed in U.S. Pat. No. 3,975,355, hereby incorporated by reference, which discloses a blend of PET with polycarbonate for making sheet to be further processed into thermoformed sheets. However, this prior art teaching is limited to nonacidic silica filler which means a pH of not less than about 6. In fact, in this earlier disclosure, because of higher concentrations it was thought that acidic silica was unsatisfactory.
In copending U.S. Ser. No. 729,739, filed Oct. 5, 1976, hereby incorporated by reference, small amounts of polycarbonate resin are physically blended into polyethylene terephthalate prior to extrusion to reduce deposits on the cooling rolls.
In copending U.S. Ser. No. 625,299 filed Oct. 23, 1975, hereby incorporated by reference, a sheet intended for hot filled applications at 150.degree.-180.degree. F. or higher is uniformly blended from about 80 to about 95 percent PET and 20 to 5 percent polycarbonate is disclosed.
In U.S. Pat. Nos. 3,956,229 and 3,975,485, there is described film and sheet formed from blends of from 60 to 85 parts of PET having an intrinsic viscosity of at least about 0.9 and 40 to 15 parts of a polycarbonate resin. Such film or sheet may be thermoformed into cook-in trays and like articles. Although such film and sheet have requisite strength and toughness to be utilized for cook-in tray applications, such sheets have a very high degree of haziness and consequently would not be suitable for applications wherein a clear sheet is desired.
In U.S. Pat. No. 3,720,732, the disclosure is limited to a biaxially oriented PET film, and discusses adding small amounts of polycarbonate to the resin to improve heat stability and slip properties. The invention of this application is directed to nonoriented amorphous sheet.
It is also known that the properties of the lower intrinsic viscosity PET resins may be modified by the addition of polycarbonate, as is described in U.S. Pat. No. 3,218,372, which also discloses an increase in the hardness, strength and electric properties of the molding material disclosed.
Glass fiber reinforced, novaculite-filled composite sheet, formed from semicrystalline PET, is disclosed in U.S. Pat. No. 3,475,140, hereby incorporated by reference. This disclosure contains an excellent discussion of all of the various forms and phases of silica, and teaches away from the use of amorphous silica in PET.